Conventionally, contents in a tubular or flat film package have been packaged by sealing the package with a heat seal (lock seal) to protect the contents during, for example, storage and transport. Regarding increase of the seal strength in case of scaling a package with a lock seal, various measures has been considered in view of, for example, a package material, a packaging machine, and packaging conditions. In view of a package material, for example, it is known to use an ethylene-based polymer such as a polyethylene and an ethylene-vinyl acetate copolymer. Among these, ionomers are widely used since they are excellent in, for example, hot tack property, low-temperature seal property, and oil resistance.
On the other hand, when it is desired that the contents be taken out of the packaging material while keeping the contents intact, with a certain degree of a lock seal property, a method for heat sealing in which a sealing section can be peeled and opened by an opening force (peelable seal) is also carried out.
For a package which can be peelably sealed after sealing with a lock seal, for example, a polymeric material including 80 to 93% by weight of an ethylene/acid ionomer and 7 to 20% by mass of a propylene/α-olefin copolymer is disclosed (see, for example, Patent Document 1). Such material can be lock sealed and peelably sealed depending on the temperature. When this polymer material is used to form a film or a layer for filling and packaging by a packaging machine, it is possible to obtain a package having both a lock seal section and a peelable seal section in one packaging material, by forming the lock seal on the back side or bottom under high temperature conditions and forming the peelable seal at the top under low temperature conditions. As the polymer material to be used for such a package, for example, a layered packaging material in which a base layer such as a polyester layer, and an ionomer layer are layered with an adhesive layer interposed therebetween, and the like are known. In this layered packaging material, it is necessary to exhibit a certain level of seal strength suitable for a peelable seal at a low-temperature sealing condition and exhibit higher seal strength suitable for a lock seal at a high-temperature sealing condition, in terms of the relationship between the sealing temperature and the seal strength.
Furthermore, in practice, it is necessary to stably form the lock seal and the peelable seal. In particular, it is required to have a temperature range in which the peelable seal can be formed stably, that is, the temperature range (plateau region) in which the seal strength of the peelable seal section does not vary significantly depending on the temperature.
As a technique regarding such a seal form, a layered film for packaging, which is provided a seal layer of a polymer composition formed of 60 to 95 parts by weight of a metal salt of an ethylene/α,β-unsaturated carboxylic acid-based copolymer and 40 to 5 parts by weight of an ethylene/α,β-unsaturated carboxylic acid ester copolymer, is disclosed (see, for example, Patent Document 2). Here, it is considered that a seal strength that is relatively low and also constant over a wide temperature range can be obtained, that is, the plateau region exhibits, and a high seal strength is exhibited under high-temperature sealing conditions.
Furthermore, a resin composition which includes (A) an ionomer including a copolymer of ethylene and α,β-unsaturated carboxylic acid and a terpolymer of ethylene, α,β-unsaturated carboxylic acid, and α,β-unsaturated carboxylic acid ester and (B) a propylene polymer, and which includes the constitutional unit derived from the α,β-unsaturated carboxylic acid ester in an amount of 1 to 4.5% by mass based on the total mass of the (A) ionomer see, for example, Patent Document 3) is known as a resin composition which exhibits high seal strength at a high temperature range in lock sealing (for example, 20 N/15 mm or more at 180° C. or higher) and a plateau region in which higher seal strength than convention is constantly exhibited over a broad temperature range without significant dependence on temperature, when a peelable seal is formed.
A resin composition which provide a lock seal and a peelable seal which exhibit a relatively low seal initiation temperature and which generate low static charge buildup is also known (see, for example, Patent Document 4).
A composition which includes a terpolymer of ethylene, alkyl (meth)acrylate (ester), and ethylene-based unsaturated carboxylic acid and an ionomer having ethylene and ethylene-based unsaturated carboxylic acid is also known as a heat sealing composition (see, for example, Patent Documents 5 and 6).
A resin composition which contains an ionomer including a copolymer of ethylene and α,β-unsaturated carboxylic acid and a terpolymer of ethylene, α,β-unsaturated carboxylic acid, and α,β-unsaturated carboxylic acid ester, and (B) a propylene polymer; wherein the melt flow rate of the ionomer at a processing temperature when a film is formed by melt T-die casting method is 50 to 250% of the MFR of the propylene polymer under the same conditions is also known as a composition which can provide, especially by T-die casting, a film suitable for a lock seal and a peelable seal (see, for example, Patent Document 7).